1. Field of the Invention
The present invention relates to an image reading apparatus, an image forming apparatus and a computer readable information recording medium.
2. Description of the Related Art
In an image reading apparatus for reading an original image, in order to guarantee reading performance, shading correction, which is well-known technique, is carried out so that variation in image data in a main scan direction is corrected. In an image reading apparatus having a plurality of image sensors disposed in a staggering manner, output characteristics of the respective image sensors may have variations. Therefore, it may be difficult to obtain uniform tone output throughout the entirety of the original image.
Further, among the plurality of image sensors, image data read by the image sensor which reads the original image first is stored in a memory temporarily so that difference in reading timing between the image sensors which read the original image in different timing respectively is corrected.
However, position shifts in main and sub-scan directions may occur in reading the original image data because of possible shifts in timing of conveying the original document, disposition of the image sensors, and so forth.
The following patent documents 1-4 discuss arts for dealing with the issues:
Patent document 1: Japanese Laid-Open Patent Application No. 2004-215011
Patent document 2: Japanese Laid-Open Patent Application No. 2004-214769
Patent document 3: Japanese Laid-Open Patent Application No. 2003-046736
Patent document 4: Japanese Laid-Open Patent Application No. 2007-150870
According to the patent document 1, in a case where one sheet of an original document is read with the original document being divided by a plurality of pick-up devices in a main scan direction, a wide area is read with the image to be read being divided in the main scan direction, for the purpose of reducing relative shifts in thus-divided read images occurring from position difference of the pick-up devices in a sub-scan direction,
That is, according to the patent document 1, the plurality of pick-up devices disposed in the main scan direction, a sub-scan part relatively driving at least one of the original image and the pick-up devices in the sub-scan direction, a speed detecting part detecting a speed in the sub-scan direction, a delay part correcting based on the detected speed a timing shift in image data between the pick-up devices because of position difference of reading viewing fields of the pick-up devices in the sub-scan direction, and a collecting part successively arranging the image data, thus corrected in the timing shifts, corresponding to an image distribution in the main scan direction, are provided.
According to the patent document 2, to precisely carry out connecting image data read by a plurality of reading sensors disposed to extend in a main scan direction in such a manner that ends of the reading sensors are overlapped one another by predetermined amounts, and the reading sensors are disposed with position difference, is directed to.
Thus, according to the patent document 2, an image pattern in a joint part (which may be referred to as an overlapped part, hereinafter) between image data read by adjacent reading sensors is recognized. Then, when the image pattern corresponds to a predetermined image pattern, a shift amount in a sub-scan direction of the respective image data read by the adjacent reading sensors is detected based on the image data in the overlapped part. The shift amount is stored. Based on the shift amount, a delay part corrects a delay time in the image data read by the upstream reading sensor in an original document conveyance direction.
According to the patent document 3, to avoid a boundary in tone or an image shift occurring at a joint part between respective sensors in an output image.
That is, according to the patent document 3, among the plurality of image sensors disposed in a staggering manner, image data of an image sensor disposed at one end in a sub-scan direction, which image data is not used for producing image data of the entirety of an original image, is stored. Based on the stored image data, image data of reading pixels of the image sensor which is disposed at the other end in the sub-scan direction and image data obtained from reading pixels at the overlapped part of the image sensor, are corrected.